1. Field of the Invention
The present invention relates to a memory device and, more particularly, to a flash memory device.
2. Description of Related Art
Generally, a semiconductor integrated circuit device performs internal operations using an externally supplied power supply voltage. Further, the semiconductor integrated circuit device generates a higher voltage than the externally supplied power voltage therein and performs the internal operations using the higher voltage. For example, in a semiconductor integrated circuit device using a higher voltage than 3.3 V, a MOS transistor of a circuit area operating with a power supply voltage (hereinafter referred to as “low voltage circuit area”) has a breakdown voltage durable to a higher voltage generated therein. In this case, a circuit area operating with a high voltage (hereinafter referred to as “high voltage circuit area”) may be directly connected to the low voltage circuit area. Although the high voltage circuit area is directly connected to the low voltage circuit area, the MOS transistor of the low voltage circuit area is not broken down by the high voltage from the high voltage circuit area. This is because the MOS transistor of the low voltage circuit area has a breakdown voltage durable to the high voltage.
As a power supply voltage drops and the integration density of semiconductor integrated circuit devices increases. MOS transistors of a low voltage circuit area includes a low voltage transistor that has a high current capacity and occupies a smaller area. Thus, breakdown voltages of the MOS transistors of the low voltage circuit area also drop. Particularly, in the case where the breakdown voltage of the low voltage transistor is lower than the high voltage used in the high voltage circuit area, the low voltage transistor of the low voltage circuit area may be broken down by the high voltage from the high voltage circuit area when the low voltage circuit area is directly connected to the high voltage circuit area.